1. Field of the Invention
This invention relates to a process for producing hyperbranched polymers. Monoethylenically unsaturated monomers are polymerized along with at least one multiethylenically unsaturated monomer at high temperatures, preferably in the substantial absence of initiators. This invention also relates to the hyperbranched polymers produced according to this method.
2. Related Background Art
Hyperbranched polymers are materials consisting of highly branched polymer chains. These branched chains often contain a large number of reactive groups which may be useful for further reactions to produce a finished product. An important property of hyperbranched polymers is their low viscosity relative to less highly branched polymers of similar molecular weight. In solvent-based systems, used to produce finished products such as coatings, use of high-molecular-weight polymers leads to undesirably high viscosity. This high viscosity may be counteracted by lowering the molecular weight of the polymer, but this may result in a finished product with inferior properties. Another means for reducing viscosity is to increase the solvent content of the system. Such an increase may be in conflict with growing environmental regulation of volatile organic compounds (VOC), such as solvents. When a hyperbranched polymer is dispersed in a solvent, the relatively low viscosity of hyperbranched polymers actually allows the solvent content of the dispersion to be decreased in order to comply with environmental requirements for low VOC content. Another important property of hyperbranched polymers is the increased durability of articles manufactured from hyperbranched polymeric resins.
Hyperbranched polymers may be classified as either dendrimers or random hyperbranched polymers. Dendrimers originate from a central location, with branching occurring as the polymer grows outward, leading to structures of relatively high symmetry. Tight control of reaction conditions and stoichiometry is required to produce dendrimers. Random hyperbranched polymers are more readily accessible from standard polymerization reactions. However, the methods employed for production of random hyperbranched polymers usually entail a separate post-polymerization step of reacting functional groups present on different polymer chains to create the branches.
Post-polymerization branching is utilized to produce hyperbranched polymers in DeLassus, S. L., et al., Macromolecules, Vol. 27, page 1307 (1994). The method of this reference employs benzocyclobutenoyl peroxide as an initiator for polymerization of styrene, and heats the resulting styrenic polymer to a high temperature in a separate step, causing the benzocyclobutene groups on different polymer chains to react. This method is limited by the use of a particular initiator and the requirement of an additional step after the initial polymerization. This reference acknowledges that attempts to make branched polystyrene in a continuous process typically lead to gel formation.
Thermally-initiated polymerization, in which a free-radical polymerization process is initiated by heating rather than by addition of initiators, has been used to prepare low molecular weight polymers from ethylenically unsaturated monomers. U.S. Pat. No. 4,414,370 describes a thermally-initiated polymerization process for preparing low molecular weight polymers in a continuous reactor, at temperatures from 235.degree. C. to 310.degree. C., with a residence time of about 2 minutes. This reference teaches that use of temperatures above 310.degree. C. leads to adverse effects on the products, for example, discoloration, oxidation, depolymerization, and side reactions. Further, this reference describes the use of a monomer mixture containing only monoethylenically unsaturated monomers, and no multifunctional monomers.
Hyperbranching in polymers formed by thermally initiated free radical polymerization may be achieved by introducing multiethylenically unsaturated monomers into a mixture of monoethylenically unsaturated monomers. This often leads to formation of highly crosslinked gels, particularly when high local concentrations of the multiethylenically unsaturated monomers form on surfaces during the polymerization reaction. These high local concentrations typically form when the multiethylenically unsaturated monomers condense on the reactor walls and on the surface of the reaction mixture. German Patent Application DE 3,026,831 describes a thermal initiation method for preparation of copolymers based on vinyl aromatics and ethylenically unsaturated carboxylic acids in which pressure pulses are applied to the reactor to remove reactants from the reactor walls, thereby minimizing gel formation. Although this reference describes preparation of polymers without gel formation using this technique, the monomer mixtures which are polymerized contain at most 1% or 2% divinyldioxane. Systems containing higher levels of diethylenically unsaturated monomer are not exemplified. At high levels of diethylenically unsaturated monomers, gelation in the bulk of the reaction mixture can also occur. Significant levels of gel in the bulk of the reaction mixture will limit both processability and solubility of the product. In addition, the polymerization reactions in this reference are carried out at temperatures between 250.degree. C. and 285.degree. C. Polymerization reactions run at temperatures higher than 285.degree. C. are not disclosed.
Preparation of hyperbranched polymers from difunctional monomers is also described in U.S. Pat. No. 5,587,446. However, in this reference the polymerization is carried out by means of a "living polymer" formed by a cationic or anionic mechanism. This is disadvantageous because monomers used for cationic or anionic polymerization must be more highly purified than those used for free radical polymerization. Consequently, most commercial polymerization of vinyl monomers is carried out using free radical polymerization. Production of a highly branched soluble polymer by a cost-effective free radical polymerization process has not been reported in the literature.
A method for prevention of gel formation in continuous free radical polymerization, by addition of solvents to the reaction mixture, is described in U.S. Pat. No. 5,508,366. This method is limited to use in reaction mixtures containing an ethylenically unsaturated monomer with at least one free hydroxyl group and an ethylenically unsaturated carboxylic acid monomer. In addition, for many applications, the solvent must be removed from the product, necessitating additional processing steps.
Continuous stirred tank reactors (CSTR) are used in commercial polymerization reactions. However, in Hamielec, A. E. and Tobita, H., "Polymerization Processes", Ullmann's Encyclopedia of Industrial Chemistry, Vol. A21, 5th Ed. (1992), it is stated that the CSTR gives more crosslinking and gel formation in free-radical polymerization than either batch reactors or continuous plug-flow reactors.
A method applicable to producing a variety of hyperbranched polymers by means of a single step consisting of free radical polymerization in a continuous reactor, without formation of highly crosslinked gels would be highly desirable.